The invention relates to a method and device for manufacturing plastic bags by transversely cutting and sealing a plastic web at spaced locations and, more particularly, to a method and device for operating a stacking device by means of a servo-motor.
The invention further relates to a method for modifying an existing bag making machine by coupling a servo-motor to the stacking device and controlling the operation of the servo-motor by means of a servo-controller.
Certain types of plastic bags are typically manufactured by drawing a tubular plastic web from a supply roll and then cutting and sealing the web transversely at spaced locations to form bags of a standard length. The formed bags are then carried by a vacuum arm assembly to a stacking device.
In the stacking device, the bags are stacked on a set of pins mounted on mobile elements, such as wicketing stands, which are attached to a conveyor. The vacuum arm assembly brings the bags being formed by a cutting and sealing bar or blade onto the pins of the stacking element in the wicketer station until a certain desired amount of bags on each element is reached. Once the desired amount of bags is reached, the "full" stacking element is moved away from the wicketer station and a new empty set of pins is brought into position whereby bags will be deposited on the set of pins on this new, empty stacking element. This procedure is repeated every time the index of the number of bags being stacked on the element positioned in the wicketer station reaches the desired amount.
In prior art devices, the pin stacking elements are arranged on a conveyor belt so that only one element is in the wicketer station at a time, and in such a position is capable of receiving bags from the vacuum arm assembly. As the element fills up with bags and reaches the desired pre-set amount, it is moved away from the wicketer station and a second element is rotated on the conveyor into the position formerly occupied by the first so that the bags will begin to accumulate on the second pin stack element. In this manner, a series of pin stack elements will be filled up with bags whereby each element has the same amount of bags.
In prior art devices having a conveyor-type system, there is a substantial amount of time required to replace a full pin stack element with an empty pin stack element. During this interval of time, the plastic web cannot be drawn. As a result, the plastic web does not advance through the bag machine to the cutting and sealing blade and bags are not formed. This avoids unnecessary production and then, consequently, a waste of bags production as the pin stack element will not be in place in the wicketer station to receive any of the bags being formed. The production of bags recommences once the empty pin stack element is in a locked position in the wicketer station. During this time, the vacuum arm assembly is rotating and, therefore, several arms will be void of bags. Generally, the movement of the conveyor requires a stoppage of bag production for an interval of time such that three arms of a typical six-arm vacuum assembly will be devoid of bags.
It is a drawback of prior art devices that the operating speed of the bag making machine is partly limited by the time factor needed for the full pin stack element to be replaced by an empty pin stack element so that the bags being produced will always be carried onto a receiving set of pins on an empty pin stack element. As the production of bags ceases during the replacement time, this results in a significant reduction in the utilization and optimization of the machine.
The amount of time for which the production of bags will cease while the conveyor moves the pin stack elements is a function of the production speed of the bag making machine. For example, if the speed of the machine is about 200 bags/minute, the machine would be required to interrupt production for at least two cycles, i.e. skip two bags, so that the pin stack element could be rotated by the conveyor. At a speed of about 300 bags/minute, a total of three machine interrupts, i.e. skipping three bags, would be required. Each interrupt translates into lost production which over the span of an hour, or even a day, is a significant amount of wasted and inefficient use of the bag making machine.
By way of example, if the desired number of bags on the pin stack element is 250 bags, the machine will interrupt for two cycles and direct the draw rolls to cease drawing the web every time the pin stack element is switched. Assuming the machine were running at an operating speed of about 300 bags/minute, the conveyor would index about 72 times. In other words, the conveyor would have to move the pin stack element 72 times in the span of one hour. Each time the pin stack element is moved, there is a stoppage of bag production and the optimization of the machine is decreased.
Moreover, the amount of interrupts of the bag machine is a function of the speed of the bag production by the machine. If the speed of production is 200 bags/minute, a total of two interrupts will be required when the plastic web will not be drawn and bags will not be cut by the cutting and sealing blade. During the time of these two interrupts, or time intervals, the conveyor will be moving the full pin stack element away from the wicketer station and bringing an empty pin stack element into the wicketer station to begin to receive bags. At a speed of 300 bags/minute, three interrupts will be required and three bags will not be produced which could have been produced by the machine. At this operating speed, a total of 216 bags (3 interrupts per switch of a pin stack element.times.72 indexing times per hour) will not be produced in every hour of operation of the bag machine. During a typical 8 hour operation of the machine, the total of lost production will be 1,728 bags. This is an even more significant disadvantage when the operating times and speeds of the machine increases.